1. Field of the Invention
The present invention relates to an ignition system for an internal-combustion engine.
2. Description of Related Art
In an ignition system used in a vehicle, voltage is applied to a primary coil of an ignition coil using an igniter, into which an ignition control circuit having a switching element and the like is incorporated. In the ignition coil, when voltage is applied to the primary coil and then the voltage is stopped in response to a pulsed spark generating signal from an ECU (electronic control unit), an induction field is formed, thereby generating high-voltage induced electromotive force (counter electromotive force) in a secondary coil. Accordingly, a spark is generated between a pair of electrodes in a spark plug, which is attached to the ignition coil.
In order to increase an amount of electricity passed through the primary coil, electrical charge, with which a capacitor is charged, is discharged into the primary coil using the ignition control circuit, in which a CDI (capacitor discharged ignition) method is employed. For example, an ignition system of multispark type, which realizes ignition performance that is the same or more than a combination of a multiple ignition method (by which an energy accumulation coil is employed, and the energy accumulation coil and the primary coil are alternately switched on and off, to discharge electrical charge) and the capacitor discharged ignition method, is disclosed in JP2811781B2 (corresponding to U.S. Pat. No. 5,056,496). In the ignition system of multispark type, a discharge time, during which electrical charge is discharged into the primary coil, is stabilized, by switching on and off the energy accumulation coil and the primary coil alternately.
However, when the primary coil is energized using the ignition control circuit, in which the CDI method is employed, a distinctive high-frequency voltage component attributed to the capacitor overlaps with output voltage of the secondary coil. In an output voltage waveform of the secondary coil shown in FIG. 7, when generation timing of a peak Vt of the high-frequency voltage component overlaps with generation timing of a peak V2max of a spark voltage V2 in the secondary coil, the peak V2max necessary to the secondary coil may not be ensured.